Power control device and related methods

ABSTRACT

A power control device for use in an AC power grid for regulating an electrical power a load that is supplied by the AC power grid consumes. The power control device has a frequency sensing functional block for detecting a deviation of the grid frequency from a nominal grid frequency and a logic functional block for performing a load adjustment process during which the power the load consumption is reduced. The load adjustment process is based at least in part on the variation of the frequency of the AC power grid. The load adjustment process is design such that for a plurality of power control devices the individual response produce a grid-wide effect that compensates imbalance between power generation and load in fashion that may reduce unwanted distortion in the AC power grid, such as flicker.

FIELD OF THE INVENTION

The present invention relates to a controller for controlling theelectrical power transferred between an AC power grid and an externaldevice.

BACKGROUND OF THE INVENTION

To ensure the reliability of an electric power grid, the utilitycontinually maintains a power reserve to compensate for a possiblefailure of electrical generation units. The power reserve is essentiallyan excess production capacity on standby. In normal conditions, thepower generation units are run at less than 100% such that a degree ofreserve power is always available. However, the maintenance of thisreserve capacity is an expensive proposition since the reserveconstitutes a resource that cannot be effectively monetized by theutility company.

An AC power grid will operate in normal conditions at a fixed frequency(usually 50 or 60 Hz). The frequency remains constant as long as thesupplied power matches the power consumed by the load. Any suddenchanges in generation or load resulting in an imbalance betweengeneration and load will lead to a frequency instability during whichthe frequency deviates from its nominal value. Large frequencyvariations are undesirable because they could lead to equipment trip oreven a system collapse.

Frequency instability events are generally caused by the sudden loss ofan electrical generation unit or by the loss of a large load and arecharacterized by a sudden frequency variation from the frequency nominalvalue.

The reserve capacity in a power grid is thus tapped when the frequencydrops below a certain level. Electrical generation units that supplypower to the grid are equipped with a speed governor. The speed governorcontinuously regulates the power output of generation units to balancethe generation with the load. Thus, when the frequency of the power gridvaries, the speed governor responds to this variation to compensate it.For example, when the frequency is higher than normal, the speedgovernor will simply lower the power generated by the generation unit(therefore reducing the amount of power supplied to the grid).Alternatively, when the frequency is lower than normal, the speedgovernor will increase the power generation. The speed governor,however, has some inherent limitations. In particular, it is slow torespond since it involves certain mechanical operations. Depending ofthe type of generation (hydraulic, gas, thermal, wind, etc.), some timeis required for the generation unit to adjust its speed up to thedesired point.

System inertia is another aspect to frequency stability of the AC powergrid. “Inertia” refers to the ability of the grid to buffer imbalances,such as excess power generation or power generation deficit and thusprevent significant and rapid frequency excursions. Any AC power gridhas a level of inherent inertia. This inherent inertia effect is theresult of the energy stored in the AC power grid that builds up orbleeds off to buffer the imbalance, depending on whether the imbalanceis the result of an excess or deficit of power generation. Most of thisenergy is the kinetic energy of the power generators. When the AC powergrid experiences a significant imbalance due to a power generationdeficit, the kinetic energy will be tapped and converted in electricityto feed the load, thus compensating the power generation deficittemporarily. As the kinetic energy bleeds off, the power generators willslow down causing the frequency to deviate from its nominal value. Therate of deviation of the frequency is thus dependent on the rate ofkinetic energy depletion. Accordingly, from the perspective of frequencystability, some level of inertia in the power grid is desirable becauseit acts as a mechanism to dampen frequency variations and thus providesmore time for slower frequency stabilization systems to become active.

Power controller devices for controlling the electrical powertransferred between an AC power grid and an electrical load are known.For example, a known power controller device is described in PCTInternational Publication No. WO 2013/177689 (hereinafter the '“689application”), the contents of which are hereby incorporated byreference. When a large number of power controllers of the typedescribed in the '689 application are installed in an electrical networkor grid, each of them is configured to respond independently to afrequency instability to reduce the amount of power that the individualelectrical load consumes. In this fashion, while each power controldevice operates autonomously, the responses are coherent andsynchronous, creating an aggregate load reduction effect, which inpractice works as a reserve, but on the load side of the AC power grid.When a frequency instability arises due to an imbalance between thepower generation and load, the load, instead of being static,dynamically responds by reducing itself to compensate fully or partiallythis imbalance.

In general, the power electronics of the power controller are configuredto lower the RMS (root mean square) voltage supplied to the electricalload, in order to lower its electrical consumption, which may involvesimply chopping-off segments of the voltage waveform. To determine theportion of the voltage waveform to chop-off to achieve the target RMSvalue, the power controller relies on the voltage zero crossings asreference points. The zero crossings are points where the voltagewaveform changes from a positive to a negative value (and/or from anegative to a positive value) and are represented by crossing of thezero value. The portion of the waveform to be chopped-off is establishedwith relation to those reference points. However, since the referencepoints are the same for each power controller (the zero crossings occurat the same instant everywhere in the AC power grid since every powercontrollers react at the same frequency change) the exact same portionof the waveform is chopped-off by every power controller. This creates adisadvantage, in certain circumstances. When the power controllerscollectively remove the same segment of the waveform, this can createunwanted distortions in the AC power grid, such as flickering.

In light of the above, there is a need in the industry for providing animproved power controller device and improved methods for operating suchpower controller device.

SUMMARY OF THE INVENTION

In accordance with a broad aspect, a power control device for use in anAC power grid for regulating an electrical power a load that is suppliedby the AC power grid consumes is provided. The power control device hasa frequency sensing functional block for detecting a deviation of thegrid frequency from a nominal grid frequency and a logic functionalblock for performing a load adjustment process during which the powerthe load consumption is reduced. The load adjustment process is based atleast in part on the variation of the frequency of the AC power grid.The load adjustment process is design such that for a plurality of powercontrol devices the individual response produce a grid-wide effect thatcompensates imbalance between power generation and load in fashion thatmay reduce unwanted distortion in the AC power grid, such as flicker.

In accordance with another broad aspect, a method for reducing aggregateload creep-up in a power distribution network which supplies anaggregate load including a plurality of individual loads controlled byrespective power control devices, the power control devices beingresponsive to a power generation deficit in the power distributionnetwork to reduce an electrical consumption of the respective loads to alevel selected according a magnitude of the imbalance, the methodincluding an act performed by each of the plurality of the power controldevices subsequent the reduction of electrical consumption to theselected level, the act including progressively reducing the electricalconsumption below the selected level to reduce a likelihood of aggregateload creep-up.

In accordance with another broad aspect, a power control device forreducing aggregate load creep-up in a power distribution network whichsupplies an aggregate load including a plurality of individual loads,the power control device configured for controlling an electricalconsumption of a respective load among the plurality of individualloads, the power control device comprising:

-   -   a. one or more processors;    -   b. a machine readable storage encoded with software for        execution by the one or more processors, the software defining        an electrical consumption control logic operative for:        -   i. in response to a power generation deficit in the power            distribution network to reduce an electrical consumption of            the respective load to a level selected according a            magnitude of the imbalance;        -   ii. subsequent the reduction of electrical consumption to            the selected level, progressively reducing the electrical            consumption below the selected level to reduce a likelihood            of aggregate load creep-up when a plurality of the power            control devices autonomously control the electrical            consumption of the respective ones of the individual loads.

In accordance with another broad aspect, a power control device for usein a power distribution network supplying electrical energy to aplurality of individual loads, the power control device for use incontrolling an electrical consumption of an individual load among theplurality of individual loads, the power control device comprising:

-   -   c. an input for receiving information identifying a presence of        a power generation deficit in the power distribution network;    -   d. a control entity;    -   e. power electronics for regulating a supply of electrical        energy from the power distribution network to the individual        load, the supply of electrical energy having sinusoidal voltage        cycles, each sinusoidal voltage cycle including a positive        half-cycle and a negative half-cycle, the control entity,        configured for:        -   i. in response to a power generation deficit in the power            distribution network, selecting a reduced non-nil electrical            consumption level for the individual load among a plurality            of possible reduced non-nil electrical consumption levels;        -   ii. determining a combination of half-cycles to block from            the electrical energy supplied to the individual load            corresponding to the selected reduced non-nil electrical            consumption level;        -   iii. control the power electronics according to the            determining to achieve the selected reduced non-nil            electrical consumption level.

In accordance with another broad aspect, a method for regulating asupply of electrical energy from a power distribution network to anindividual load, the supply of electrical energy having sinusoidalvoltage cycles, each sinusoidal voltage cycle including a positivehalf-cycle and a negative half-cycle, the method comprising:

-   -   f. accessing information identifying a presence of a power        generation deficit in the power distribution network;    -   g. executing software by one or more processors to implement a        control entity, configured for:        -   i. in response to a power generation deficit in the power            distribution network, selecting a reduced non-nil electrical            consumption level for the individual load among a plurality            of possible reduced non-nil electrical consumption levels;        -   ii. determining a combination of half-cycles to block from            the electrical energy supplied to the individual load            corresponding to the selected reduced non-nil electrical            consumption level;        -   iii. control power electronics in an electrical energy            supply path from the power distribution network to the            individual load, according to the determining to achieve the            selected reduced non-nil electrical consumption level.

In accordance with another broad aspect, a power control device for usein a power distribution network supplying electrical energy to aplurality of individual loads, the power control device for use incontrolling an electrical consumption of an individual load among theplurality of individual loads, the power control device comprising:

-   -   h. an input for receiving information identifying a presence of        a power generation deficit in the power distribution network;    -   i. a control entity;    -   j. power electronics for regulating a supply of electrical        energy from the power distribution network to the individual        load, the supply of electrical energy having sinusoidal voltage        cycles, each sinusoidal voltage cycle including a positive        half-cycle and a negative half-cycle, the control entity,        configured for:        -   i. in response to a power generation deficit in the power            distribution network, selecting a reduced non-nil electrical            consumption level for the individual load among a plurality            of possible reduced non-nil electrical consumption levels;        -   ii. control the power electronics to reduce an RMS voltage            of the supply of electrical energy to the individual load to            achieve the selected reduced non-nil electrical consumption            level without introducing a DC component in the supply of            electrical energy to the individual load.

In accordance with another broad aspect, a method for regulating asupply of electrical energy from a power distribution network to anindividual load, the supply of electrical energy having sinusoidalvoltage cycles, each sinusoidal voltage cycle including a positivehalf-cycle and a negative half-cycle, the method comprising:

-   -   k. accessing information identifying a presence of a power        generation deficit in the power distribution network;    -   l. executing software by one or more processors to implement a        control entity, the control entity, configured for:        -   i. in response to a power generation deficit in the power            distribution network, selecting a reduced non-nil electrical            consumption level for the individual load among a plurality            of possible reduced non-nil electrical consumption levels;        -   ii. control the power electronics to reduce an RMS voltage            of the supply of electrical energy to the individual load to            achieve the selected reduced non-nil electrical consumption            level without introducing a DC component in the supply of            electrical energy to the individual load.

In accordance with another broad aspect, a power control device forcontrolling an electrical consumption of an electrical load supplied bya power distribution network, the power control device comprising:

-   -   m. an input for receiving information identifying a presence of        a power generation deficit in the power distribution network;    -   n. a control entity;    -   o. power electronics for regulating a supply of electrical        energy from the power distribution network to the individual        load, the supply of electrical energy having sinusoidal voltage        cycles, each sinusoidal voltage cycle including a positive        half-cycle and a negative half-cycle, the control entity,        configured for:        -   i. in response to a power generation deficit in the power            distribution network, selecting a reduced non-nil electrical            consumption level for the individual load among a plurality            of possible reduced non-nil electrical consumption levels;        -   ii. control the power electronics to reduce an RMS voltage            of the supply of electrical energy to the individual load to            achieve the selected reduced non-nil electrical consumption            level while maintaining flicker in the supply of electrical            energy to a level acceptable as defined in anyone of            International Electrotechnical Standards IEC 6100-3-3, IEC            6100-3-11 and/or IEC 6100-3-12.

In accordance with another broad aspect, a method for controlling anelectrical consumption of an electrical load supplied by a powerdistribution network, the method comprising:

-   -   p. accessing information identifying a presence of a power        generation deficit in the power distribution network;    -   q. an act of regulating a supply of electrical energy from the        power distribution network to the individual load, the supply of        electrical energy having sinusoidal voltage cycles, each        sinusoidal voltage cycle including a positive half-cycle and a        negative half-cycle, the control entity, the act of regulating        including:        -   i. in response to a power generation deficit in the power            distribution network, selecting a reduced non-nil electrical            consumption level for the individual load among a plurality            of possible reduced non-nil electrical consumption levels;        -   ii. reducing an RMS voltage of the supply of electrical            energy to the individual load to achieve the selected            reduced non-nil electrical consumption level while            maintaining flicker in the supply of electrical energy to a            level acceptable as defined in anyone of International            Electrotechnical Standards IEC 6100-3-3, IEC 6100-3-11            and/or IEC 6100-3-12.

These and other aspects of the invention will now become apparent tothose of ordinary skill in the art upon review of the followingdescription of embodiments of the invention in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A detailed description of non-limiting examples of implementation of thepresent invention is provided herein below with reference to thefollowing drawings, in which:

FIG. 1 shows an example of an electrical power grid, illustrating thepower generation side and the distributed load side of the power grid;

FIG. 2 is a bloc diagram of a power-control device in accordance with anon-limiting example of implementation of the invention used to regulatethe electrical power that a load is allowed to consume, based on the ACfrequency;

FIG. 3 is a more detailed bloc diagram of the power control device shownin FIG. 2;

FIG. 4 is a flow chart of the process implemented by the power controldevice of FIG. 3 for controlling an electrical load;

FIG. 5 is a flow chart of the process implemented by the power controldevice of FIG. 3 for determining the electrical energy consumptionadjustment strategy;

FIG. 6 is an example of a lookup table for looking up a controlled loadamount based on frequency deviation;

FIGS. 7A and 7B are example voltage waveforms supplied by the powercontrol device to the electrical load;

FIG. 8 is an example of a table for a modulation strategy for the powercontroller;

FIG. 9 is an example of a time schedule power adjustment strategy forthe power controller;

FIG. 10 is an example of a low voltage transformer connected to aplurality of power controllers of the type illustrated in FIG. 3;

FIG. 11 is an example of a medium voltage transformer connected to a lowvoltage transformer

FIG. 12 is an example of a low voltage transformer supplying power to aplurality of dwellings where the low voltage transformer is respectivelyconnect to a plurality of power controllers associated with a selectnumber of the dwellings;

FIG. 13 is an example of a set of the power control devices of the typeillustrated in FIG. 3;

FIG. 14 illustrates examples of voltage waveforms supplied by the setpower control devices of FIG. 13 to electric loads;

FIGS. 15A and 15B are example voltage waveforms supplied by the powercontrol device to the electrical load;

FIG. 16 is an example of a lookup table for looking up delay times andload reduction times for a specific controlled load;

FIG. 17 is an example of a lookup table for looking up the number ofcycles to reduce for a specific controlled load;

FIG. 18 is an example of a lookup table for looking up sequences ofcycles to reduce for a specific controlled load;

FIG. 19 illustrates examples of voltage waveforms supplied by the setpower control devices of FIG. 16 to electric loads; and

FIGS. 20A and 20B illustrate examples of rectified DC signals.

In the drawings, embodiments of the invention are illustrated by way ofexample. It is to be expressly understood that the description anddrawings are only for purposes of illustration and as an aid tounderstanding, and are not intended to be a definition of the limits ofthe invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

To facilitate the description, any reference numeral designating anelement in one figure will designate the same element if used in anyother figure. In describing the embodiments, specific terminology isused but the invention is not intended to be limited to the specificterms so selected.

FIG. 1 shows an AC power grid. Electricity is generated at a power plant10 and is transmitted over high-voltage transmission lines 12 to avoltage down step station 14. The voltage down step station 14 lowersthe electrical voltage (via transformers for example) such that it maybe distributed to households 16 and industrial buildings 18 viaresidential distribution lines 20.

In a first example of implementation, the present invention provides apower controller 32 that can regulate the amount of electrical energythat household appliances or industrial equipment are allowed toconsume. By using a sufficient number of such power control devices, asignificant portion of the grid load is controllable and can thusprovide a significant effect on the overall power demand.

FIG. 2 is a diagram of the power controller 32, showing the powercontroller 32 connected to an AC supply 30 (which is the AC power grid)and to an electrical load 34. The power controller 32 monitors thefrequency of the AC supply 30 via the power supply connection to the ACsupply. If the frequency varies from its nominal value, the powercontroller 32 reacts to adjust the electrical consumption of the load34.

With reference to FIG. 3, a more detailed block diagram showing thedifferent components of the power control device of FIG. 2 is shown. Thepower controller 32 is computer based and uses software to interpret theAC frequency and implement the desired load regulation strategy. Thepower controller 32 has an input/output interface 40, a CPU 42, amachine-readable storage 44 and power electronics 46. Signalsrepresentative of the AC frequency, which are sensed via the powersupply connection between the electrical load 34 and the AC supply 30are communicated to the power controller 32 via the input/outputinterface 40. The input/output interface 40 reads the frequencyinformation, digitizes it and makes it available to the CPU 42 forprocessing. Although the term “CPU” is used throughout this document, itis appreciated that any suitable processor for executing logic and/orprogram code setting out the various functions, procedures and/ormethods described in this document may be used. Such examples include amicroprocessor, digital signal processors (DSP), application-specificintegrated circuit (ASIC), field-programmable gate array (FPGA), etc.

The machine-readable storage (memory) 44 is encoded with softwareexecuted by the CPU 42. The software implements the load regulationstrategy. The I/O interface 40 outputs control signals that aregenerated by the software to command power electronics 46 for performingthe actual power control. The power electronics 46 typically wouldinclude thyristors or power transistors that can lower the RMS (rootmean square) voltage and the energy supplied to the electrical load 34.The power electronics 46 can simply chop segments of the voltage wave toeffectively lower the RMS supply voltage hence, the amount of power theload consumes. The power electronics 46 would typically includeelectronic components that can conduct current in either direction(i.e., bidirectional) when triggered (turned on). A TRIAC, also known asa bidirectional triode thyristor or bilateral triode thyristor, is anexample of such bidirectional electronic component.

The control signals output from the I/O interface 40 convey informationindicating the amount of power reduction desired. In response to thesecontrol signals the power electronics 46 control the AC voltage waveaccordingly.

Examples of Loads that Can be Controlled

The loads that are the most suitable to be controlled, but notrestricted to, by the power controller 32 are resistive loads. The powerconsumed by a resistive load can be adjusted by varying the supplyvoltage to provide a continuous range of power consumption regulation.

In one specific example of implementation, the electrical load 34 is awater heater in a dwelling. If a decrease in the power consumption ofthe water heater is necessary, the power electronics 46 will reduce thesupply RMS voltage according to the programmed control strategy toobtain the desired power consumption reduction. The decrease in powerlevel can be enforced for a short period of time (for example, ten tothirty minutes) to avoid an excessive cooling of the water load. In thisparticular example, it is unlikely that the power consumption reductionwill affect in a major fashion the functionality of the apparatus andwould be almost imperceptible to the end user. The large thermal mass ofthe water load (assuming that it is at the set point temperature whenthe load reduction was initiated) may reduce the water temperature by afew degrees and be virtually unnoticeable by the end user. As will befurther discussed below, such an effect would be even less perceptibleif the power control occurs at times during which the water heater isnot being heavily used, such as during the night.

Another example of a load suitable to be controlled is a heating systemin a commercial building or a home. In such embodiments, if it isnecessary to decrease the power consumption of the load, the powercontroller 32 can instruct the heating system to reduce the consumedpower for a period of thirty minutes, for example. During such a controlperiod, it can be understood that the overall temperature of thecommercial building or home may not vary greatly due to the thermalinertia of the building. Hence, such a variation to the end user wouldonce again be small. Note that the heating system may be of resistivenature (electrical heating elements) that can be regulated via the powerelectronics 46.

Yet another example of a load suitable to be controlled would be anindustrial facility implementing a process that requires a significantamount of electrical energy but whose power consumption can be reducedto some degree over a certain period of time without any major drawbackon the process itself. An example is an aluminum smelter.

Another example of an apparatus to which a power controller can beconnected is an electric oven for food preparation purposes. Forexample, if the oven is set to operate at a temperature of 450° F., areduction in power supplied to the oven for a short period of time willnot drastically change the temperature of the oven. The oven control canbe similar to the heating system control described earlier.

Yet another example of the load that could be controlled by the powercontroller 32 is a electric clothes dryer. The clothes dryer includes aheating system that can be regulated in a continuous fashion asdescribed earlier. In a period of usage, the power controller 32 canreduce the amount of electrical power made available to the heatingsystem of the dryer. From the point of view of the end-user, thiselectrical power reduction will translate into a longer drying time.

Electrical Consumption Regulation Strategy

Generally, the electrical consumption regulation strategy has two mainphases. The initial phase is a response to an observed frequencydecrease. The purpose of this response is to adjust the amount ofelectrical energy consumed by the electrical load 34. This phaseincludes determining the degree of electrical energy consumptionreduction necessary and the process to implement this reduction. Thesecond phase is the restoration phase. During the restoration phase, theelectrical consumption of the load 34 is restored.

FIG. 4 is a flow chart of an example of the process implemented by thepower controller 32. After the power controller 32 is in an active state(generally represented by a “Start” condition at step 50), the logic ofthe power controller 32 proceeds to step 52 where the AC frequency ismeasured to determine if the electrical consumption of the load 34 needsto be adjusted.

Measuring AC Frequency

The purpose of the AC frequency assessment is to detect an imbalancebetween the generation side of the grid and the load side thereof, whichis reflected by the frequency deviation. Typically, the larger thedeviation the larger the imbalance. The output of step 52 is thus afrequency value. Since the power controller 32 performs digital dataprocessing, the frequency value is preferably generated in a digitalformat. Any suitable methodology can be used to convert the AC analogwaveform into digital frequency information. A possible refinement is toperform several frequency measurements and to compound thosemeasurements into a single representative value, such as by averagingthem. Specifically, the power controller 32 is programmed to acquireover a predetermined period of time a frequency measurement, which isstored in the memory of the power controller 32. In a specific example,a frequency measurement can be made over a 100 ms interval, but thisvalue can vary in various implementations of the invention.

The memory of the power controller 32 keeps a certain number offrequency measurements. As a new measurement becomes available, it isstored in the memory and the oldest measurement is overwritten. All thefrequency values that are stored in the memory are averaged as a newfrequency measurement becomes available. The average measurementsmoothes out short-term frequency variations that may not be indicativeof the grid frequency instability.

Note that instead of averaging the frequency measurements, other ways toblend this data into a single representative value is possible.

In addition to computing a frequency measurement, which reflects thecurrent frequency value, step 52 optionally computes the rate ofvariation of the frequency. The rate of variation of the frequency is anindicator of the AC power grid stability; the faster the frequencydiminishes the greater the risk of AC power grid collapse. Accordingly,the rate of variation of the frequency can be used as a factor to tailorthe electrical consumption reduction to restore the balance between thegeneration side and the load side of the AC power grid, or at leastprevent further balance deterioration.

Several possibilities exist to determine the rate of frequencyvariation. One is to measure the rate of frequency variation from thenominal AC power grid frequency versus time. In other words, the processcomputes the first order derivative of the frequency change versus time.Practically, this measure reflects the speed at which the frequencyvaries.

Another possibility to determine the rate of frequency variation is tomeasure the rate at which the rate of variation of the frequency itselfvaries. This measurement, which is the derivative of the rate offrequency variation versus time, reflects the acceleration of thefrequency variation.

The rate of frequency variation versus time is computed on the basis ofconsecutive frequency measurements stored in the memory and the timeintervals separating the frequency measurements. If desired, furthercomputations can then be performed to derive the acceleration of thefrequency variation from the speed of frequency variation.

The targeted electrical energy consumption is determined on the basis ofseveral factors, namely the current value of the frequency, the rate atwhich the frequency varies, the magnitude of the acceleration of thefrequency variation or a combination of the aforementioned. Otherfactors can also be used to further fine-tune the electrical energyconsumption regulation.

The '689 application further describes how frequency typically varies inan AC power grid, when a generation unit is lost, and the reader isdirected to the '689 application for further information.

The power controller 32 implements decision logic based on thecompounded frequency measurement and also the rate of frequencyvariation in order to determine the electrical energy consumption asrepresented by step 54. Subsequently, the power controller 32 sends acorresponding command to the power electronics 46 (via control signals,for example) as represented by step 56.

Step 54 of the process thus uses the frequency measurement andoptionally the rate of variation of the frequency as an input indetermining if an adjustment (i.e., reduction or increase) of theelectrical consumption is required. Step 54 may optionally determine thestrategy to be employed (when different strategies can be used). In someembodiments, the power controller 32 may be programmed with a specificstrategy to be employed and at step 54 after the amount of adjustment ofthe electrical consumption is determined the adjustment strategy isapplied. In instances, when the AC power grid is stable and thefrequency is within a nominal acceptable range, the processing at step54 determines that no electrical consumption adjustment is necessary andno further action takes place. This processing loop constantly repeatsto provide a continuous monitoring of the grid frequency stability. Assuch, when the frequency of the AC power grid is not within a nominalacceptable range, the power controller 32 may regularly determine theelectrical consumption level and/or strategy to be employed, based onthe frequency instability.

When a large number of power controllers 32 is installed in theelectrical network or grid, each of them responds independently to thefrequency instability event. Since the responses are coherent andpredictable, they all add up to a combined load reduction that has agrid-wide effect.

Determining the Adjustment of Electrical Energy Consumption

At step 54, in the process of determining the electrical energyconsumption adjustment strategy, the first step is to determine thedegree of electrical energy consumption adjustment required, as is shownby step 102 in FIG. 5.

In an embodiment, the memory 44 of the power controller 32 stores atable mapping current frequency to respective power consumption valuesaccording to a linear relationship. Once the frequency deviation hasbeen computed, the table is selected from the memory 44 and based on theconsumption value in the table associated with that frequency deviation,the electrical consumption of the electrical load 34 is adjusted (i.e.,increased or reduced) by an amount such that the consumption of theelectrical load 34 generally corresponds with the consumption value inthe table for the calculated frequency deviation. FIG. 6 illustrates anexample table 1000 that shows the value of the controlled load (thepercentage of rated value) for various corresponding frequency deviationvalues. The logic of the power controller 32 based on the determinedfrequency deviation may use this table 1000 to look-up a correspondingcontrolled load value. Although table 1000 illustrates in the secondcolumn the controlled load (i.e., the percentage of rated value), inother cases the table 1000 may list the percentage of reduction of theload. It is appreciated that the values given in table 1000 are forillustrative purposes only and that other suitable values may be used invarious examples of implementation.

Instead of using a table look-up operation, an algorithm or formula canbe programmed to compute directly the power consumption of theelectrical load 34, based on the frequency deviation.

In other words, the power controller 32 is programmed to compute thepower consumption of the electrical load 34, based on the frequencydeviation. The programmed response of the power controller 32 to computethe power consumption of the electrical load 34 may be determined invarious ways. For example, the power consumption of the electrical load34 may be determined based on the frequency variation versus time, themagnitude of the acceleration of the frequency variation and/ordetermined based on any other suitable technique.

The '689 application describes various techniques for the adjustment ofthe power consumption of the electrical load 34, and the reader isdirected to the '689 application for further information.

One technique for adjusting the electrical consumption of the electricalload 34 is based on the magnitude of the frequency imbalance event. FIG.9 illustrates an example of how the electrical energy consumption of theelectrical load 34 may be adjusted in response to an occurrence of afrequency imbalance event. For instance, when the magnitude of thefrequency imbalance event (e.g., the magnitude of the current frequencyvalue, the magnitude of the rate of frequency variation versus time, orthe magnitude of the acceleration of the frequency variation) is above athreshold, the power control 32 may respond to adjust the load accordingto the strategy shown in FIG. 9. In other words, based on the determinedvalue of the controlled load (the percentage of rated value) or if themagnitude of the reduction of the controlled load is above a thresholdor within a range, a specific response for adjusting the electricalconsumption of the electrical load 34 may be used. Different determinedvalue of the controlled load (the percentage of rated value) may havedifferent responses programmed into the memory 44 of the powercontroller 32. As shown, in response to a frequency imbalance thecontrolled load is reduced by a first reduction R1, the first reductionR1 in this embodiment is based upon the frequency deviation and isproportional to frequency deviation until the nadir N of the frequencydeviation is reached. In other words, the power controller 32 may bedesigned to continuously or incrementally reduce the controlled load (%of the rated value) until the nadir N of the frequency deviation isreached, at which point the total amount of this reduction is defined bythe value R1. The amount of the first reduction R1 may be determinedaccording to the degree of electrical energy consumption adjustmentrequired (e.g., as determined at step 102, as discussed elsewhere inthis document). In other words, the reduction R1 may be associated withthe value of the controlled load (the percentage of rated value). Forexample, if it was determined that the electric load 34 should operateat 75% of the rated value at step 102, the reduction R1 could bedetermined to be a 25% reduction of the rated value. The reduction R1occurs over a first time period T1—this first time period T1 istypically not programmed into the power controller 32 but is the timethat it takes the reduction R1 to occur. After the first reduction R1,the controlled load is then further reduced by a second reduction R2 inthe percentage of the rated value in a second specific time period T2(e.g., 15 minutes). The second specific time period T2 is typicallylonger in duration than the first time period T1. For example, T1 may bein the order of magnitude of seconds (or less) while T2 may be in theorder of magnitude of minutes. The second specific timer period T2 wouldtypically be programmed into the power controller 32. The purpose of thereduction R2 during this second time period T2 is to manage thecontrolled load (% of rated value) in order to maintain the load, asseen by the generation side, as being generally constant. It has beenobserved that if the controlled load is maintained constant, the load asseen by the generation side, will not stay constant but would insteadcreep up. This is a network effect and results from the fact that loadsthat would normally switch off may have to remain on for a longer periodof time since they are operating at a reduced percentage of the ratedvalue. The various loads in the power network are characterized by acollective inertia; for instance, in the case of water heaters, thatinertia is of thermal nature owing to the thermal energy storedindividually by each water heater. The larger the inertia, the lesserthe network effect; in other words, the absolute value of the slope S2can be lessened while still maintaining the electrical energyconsumption at the network level constant. In contrast, the lesser theinertia the larger the network effect, which requires an increasedabsolute value of the slope S2.

After the second specific time period T2, the controlled load isincreased by an increase R3 in a third time period T3 (e.g., 90minutes), while the power generation supply (e.g., at the power plant10) is also increased over the third time period T3. The third specifictime period T3 is typically longer in duration than the first timeperiod T1 and second time period T2, and may be in the order ofmagnitude of minutes or hours. The increase R3 takes place during arestoration phase in which the controlled loads in the power network arebrought back to 100% of the rated value while the power generationsupply is also adjusted to meet the load demands. The value of thirdtime period T3 is chosen to smooth the pick-up demand of the load. Thethird specific timer period T3 would typically be programmed into thepower controller 32. The adjustment time periods T2 and T3 shown are forillustration purposes and the power controller 32 may be designed tohave various ranges of time periods for decreasing and then increasingthe controlled load.

Each adjustment (e.g., the first reduction R1, the second reduction R2and the increase R3) is based on a combination or frequency and timeparameters. The transition between R1 and R2 is frequency dependent; theR1 phase exists as long as the frequency is decreasing as a result ofthe unbalance between power generation and load in the powerdistribution network. The R2 phase is engaged when the frequency hasreached the nadir N. The R2 phase is time-dependent and it is maintainedover a predetermined time period. In the example shown the time windowT2 is of 15 minutes, however other values can be used without departingfrom the spirit of the invention.

R3 is also time-dependent, although the frequency is monitored to ensurethat it remains stable as the load is progressively being brought up. Inthe example shown the time window T3 is of 90 minutes, however othervalues can be used without departing from the spirit of the invention.

Since R2 and R3 are determined by the load inertia, the power controllermay be programmed with different slopes (or curves) S2 and S3 dependingon the particular load that is going to be controlled. For instance, ifthe power controller will regulate the electrical consumption of a waterheater it may be programmed to implement a slope S2 and S3 determinedaccording to the inertia behavior of water heaters. However, when thepower controller is designed to control a different load, which has moreinertia or less inertia, the value of the slope S2 and S3 would bedifferent.

A specific and non-limiting example will now be discussed where theelectric load 34 is a water heater. The water heater typically turns theelectric load 34 on or off based on the measured temperature of thewater in the tank of the water heater in relation to the set temperatureof the water heater. After the water in the tank of the water heater isabove the set temperature level, the electric load 34 is switched off.The thermal inertia of the water heater including the rate at which hotwater is being discharged from the water heater determines the rate atwhich the water heater will switch on and off.

The reduction R2 is selected to take into account the effect of waterheaters staying at an on state for a longer period of time when thepower is reduced by the power controller, decreasing the diversity amongseveral water heaters. Stated otherwise, the reduction R2 takes intoconsideration the fact that the electrical consumption of the waterheater is reduced in response to a frequency imbalance and it would haveto remain in the on state for a longer period of time to heat the waterback up to the set temperature, as the load is operating at a reducedpercentage of the rated value. In other cases, the reduction R2 may alsobe designed to take into consideration that in the power network at anygiven time there are a plurality of water heaters where some of thewater heaters are on and some are off and that the water heaters mayturn on and turn off at various times.

Accordingly, the slope (or curves) S2 and S3 may be based on the type ofappliance, and more specifically on the inertia of the appliance, thatthe power controller manages.

One option is to program the power controller at the manufacturing stageto operate with a predetermined slopes (or curves) S2 or S3 with theunderstanding that the power controller will be used with the loadcorresponding to that particular S2 and S3 value. Objectively, thisapproach creates a logistical burden since the manufacturer may need toproduce different types of power controllers to suit the various typesof loads that will be controlled. This is less of a problem when thepower controller will be integrated into a new appliance; under thatscenario, the power controller may be programmed with the particularslopes S2 and S3 as the appliance is assembled. However, in the case ofa retrofit, when a power controller is installed to control theelectrical consumption of an already existing appliance then the correctpower controller needs to be selected with a slopes S2 and S3 thatmatches the appliance.

Alternatively, the power controller can be provided with an automaticdiscovery protocol designed to determine the characteristics of theappliance and on the basis of those characteristics it can select aslopes S2 and S3 that matches the appliance. An example of an automaticdiscovery protocol is an algorithm, which determines, based on theelectrical consumption of the appliance what its inertia is. Here, theobjective is not to derive a precise value of the inertia; rather it isto identify a category among a number of categories in which theappliance can be placed and on the basis of the selected category todetermine the slopes S2 and S3 (different slopes being associated withdifferent categories). A simple example is to provide an algorithm thatlooks at the frequency at which the appliance switches on and off whenthere is no under frequency event. The faster the rate, the lesser theinertia of the appliance will be. Note that this approach also takesinto consideration how the appliance is being used, in addition to theinherent nature of the appliance. Accordingly, the algorithm can selectthe slopes S2 and S3 that matches the type of appliance and also the waythe appliance is being used (heavy use, light use or medium use).

In view of this technique above, the power controller 32 may beprogrammed to compute the power consumption of the electrical load 34(or the adjustment of the power consumption), based on frequencyinstability (e.g., current frequency value, the rate of frequencyvariation versus time, the acceleration of the frequency variation) andapply one or more responses to reduce the power consumption of theelectrical load 34 followed by a restoration phase.

The Power Reduction Process

Once the amount of adjustment of the electrical consumption by theelectrical load 34 is determined (step 102), the next step is todetermine a specific electrical consumption process to achieve thedesired adjustment of the electrical consumption by the electrical load34, this is shown by step 104 in FIG. 5.

As noted above, the power electronics 46 are configured such that theycan lower the RMS voltage supplied to the electrical load 34, which mayinclude the power electronics 46 reducing segments of the voltage waveto zero. FIGS. 7A and 7B illustrate examples of several cycles ofvoltage waveforms, where the voltage waveform in FIG. 7A does not haveany reduction and the voltage waveform in FIG. 7B has severalhalf-cycles reduced to zero volts (0 V). In other words, the voltagewaveform in FIG. 7A may be applied to the electric load 34 when it isdesirable to operate at 100% of the rated value (i.e., no reduction) andthe voltage waveform in FIG. 7B may be applied to the electric load 34when it is desirable to operate below 100% of the rated value, namely,at 80% of the rated value (i.e., 20% reduction).

Half-cycles of the voltage waveform supplied to the electrical load 34may be reduced to zero volts (0 V) depending on the desired percentageof the rated value of the electrical load 34. For instance, if onehalf-cycle is reduced to zero in a two and a half cycle window (i.e.,one half-cycle every five half-cycles is reduced to zero), then theelectric load would then operate at 80% of the rated value (i.e., 20%reduction)—this is illustrated in FIG. 7B. It should be appreciated thatto prevent the creation of a DC component it is desirable to balance thenumber of positive and negative half-cycles reduced in a given timeperiod. This balancing may be done by reducing positive and negativehalf-cycles in an alternating manner (e.g., after a positive half-cycleis reduced a negative half-cycle is reduced next and so forth) or byhaving the same number of positive and negative half-cycles reduced in agiven time period. By way of another example, if one half-cycle isreduced to zero in a one cycle window, then the electric load would thenoperate at 50% of the rated value (i.e., 50% reduction) Similarly, ifone half-cycle is reduced to zero in a three cycle window, then theelectric load would then operate at 83.33% of the rated value (i.e.,16.66% reduction). In other words, by reducing some half-cycles of thevoltage waveform supplied to the electrical load 34 in a specific window(i.e., number of cycles) the percentage of the rated value of theelectrical load 34 can be controlled accordingly.

Since the reference point in the waveform for reducing the voltage is azero crossing, all the power controllers in the power network areeffectively synchronized in their operation since the zero crossingsoccur at the same moment for each power controller. In addition, sincethe power controllers all read the same frequency, they will typicallycompute the same degree consumption reduction. Accordingly, the samepart of the waveform will typically be chopped-off by each powercontroller potentially creating unwanted mass effects in the AC powergrid.

To overcome this problem, the individual responses of the power controldevices can be managed to avoid unwanted distortions. One form ofmanagement is to randomize the responses by the power controllers. Inother words, the power controllers may be designed such that not all ofthe power controllers in the network operate exactly in the same way inresponse to frequency instability events. Optionally, the randomizationis determined by taking into consideration three requirements. The firstis to maintain a coherent response of the power control devices. Inother words, the individual responses should still work cohesively witheach other to produce a desired grid-wide effect that compensates theimbalance between power generation and load. Accordingly, the individualresponses cannot be dispersed time-wise too much, otherwise thecompensation to the imbalance between power generation and load, will besluggish. The second is to disperse the responses time-wise sufficientlysuch as to reduce unwanted distortions in the AC power grid. The thirdis to balance the number of positive and negative half-cycles reduced ina given time period in order to avoid DC component.

One strategy for adjusting the electrical consumption is to program thepower controller 32 to reduce half-cycles of the voltage waveformaccording to a modulation strategy. The modulation strategy may beimplemented by storing values (e.g., half cycle on or half cycle off) ina look-up table for a specific controlled load (% of the rated value)over a window of time. It should be appreciated that the look-up tablemay specify which positive and negative half-cycles to reduce. Incontrast to the technique discussed above in relation to FIG. 7B, thewindow time can be increased to a larger period of time (e.g., 600half-cycles) and a specific half-cycle is removed according to the tableindication. This table may specify which positive and negativehalf-cycles to reduce. FIG. 8 illustrates a graph based on thedetermined controlled load (% of the rated value) indicating for eachhalf cycle in the window whether they are on or off. As shown, theblack-colored sections of the table indicate that a half-cycle is offwhile a white-colored section indicates that a half-cycle is on.

The power controller 32 counts the half-cycles of the voltage waveform,such that the voltage waveform that is applied to the load is modifiedaccording to FIG. 8. The counter may be in the form of an index which isincremented each half cycle. Based on the index a particular half cycleand also based on the percentage of the controlled load the powercontroller 32 can determine whether that particular half cycle is on oris off. Once the half cycle count reaches the end of the time window, inthis case reaches the 600 value, then the FIG. 8 is repeated from thestart. In other words, the half cycle index is reset to 0 and theoperation starts again.

Note that the index may not necessarily start at the 0 mark. The tableof FIG. 8 can be entered at any index value and the operation started atthat point. The selection of the entry point can be made by using somesort of a random or pseudo-random number generator. In this fashion, adegree of randomization can be achieved for each power controller 32such as to avoid the same half-cycles being on or off at the same time.This characteristic is described in greater detail below.

It is appreciated that the modulation strategy may be applied to the ACvoltage waveform to reduce the RMS value of the AC voltage waveform suchthat the desired adjustment of controlled load (% of the rated value) asdetermined in step 102 is achieved. The modulation strategy may bestored in a look-up table for various values of the amount of thecontrolled load (% of the rated value). The modulation strategy may bestored as a formula or algorithm The modulation strategy may includereducing various half-cycles of the AC voltage waveform in a specificnumber of half-cycles based on the desired reduction of the controlledload (% of the rated value).

Additionally, the modulation strategy in the graph of FIG. 8 may be usedwith the response strategy shown in FIG. 9. In such case, based on afrequency instability event the power controller 32 may first determinethe amount of frequency instability (step 52) and then respondsaccording to the amount of frequency instability. The one or morereductions for adjusting the electrical energy consumption may depend onthe magnitude of the frequency instability. As such, the powercontroller 32 may be programmed with various responses that may be useddepending on the magnitude of the measured frequency instability. Afterthe occurrence of the frequency instability event, the adjustment of theamount of controlled load (% of rated value) may follow one or morereductions (e.g., as shown in FIG. 9), where the strategy for reducingthe controlled load (% of the rated value) may be done according to themodulation strategy to achieve the desired reduction in the controlledload (% of the rated value). For instance, as the reductions in FIG. 9are applied, the modulation strategy to achieve the desired value of thecontrolled load (% of the rated value) is obtained from the table inFIG. 8, or similar

One technique to randomize the electrical energy consumption adjustmentwhen there is a plurality of power controllers in the power network isthat the plurality of power controllers may be programmed withnon-identical responses. In other words, the plurality of powercontrollers may have modulation strategy tables of the type shown intable in FIG. 8 for adjusting the controlled load but where at leastsome of the power controllers have modulation strategy tables thatdiffer from one another. Another technique to randomize the electricalenergy consumption adjustment is to start the modulation strategy intable of FIG. 8 at a random start point in response to a frequencyinstability event. The random start point of the modulation strategy maybe accomplished by a random number generator that determines the startpoint (e.g., a random number generator between 1 and 600 and themodulation strategy in FIG. 8 starts at the half-cycle corresponding tothe randomly generated number). Alternatively, the power controller 32may be programmed with a quasi-random number (i.e., a seed or startindex value) that the counter will start its count in response to afrequency instability event, and when there is a plurality of powercontrollers in the network at least some of them have differentquasi-random numbers.

It is appreciated that the modulation strategies to reduce half-cyclesof the voltage waveform applied to the load 34 may be designed invarious ways. As discussed above, the modulation strategy among aplurality of power controllers may be designed to have some variance inthe modulation strategy employed among the plurality of powercontrollers (e.g., non-identical responses) in order to introduce someform of randomness in the power adjustment process, keeping in mind thatthe overall load as seen by the grid should follow the electrical energyconsumption target. The modulation strategy may also be designed to takeinto account flicker.

When the power controller 32 makes adjustments to electrical load 34,this causes voltage fluctuations, those voltage fluctuations affect thecontrolled load and also electrical equipment which is connected to thesame power supply as the controlled load. Since the controlled loadtends to draw large amperage, when a half cycle is cut off the voltageof the power supply (the power distribution panel in a dwelling forexample) slightly increases. Those voltage fluctuations are usuallyreferred to as “flicker”. When lights in the dwelling are supplied byvoltage which carries some degree of “flicker”, an undesirable visualeffect can ensue. In general, visual flicker is a visible change inbrightness of a lamp typically due to rapid fluctuations in the voltageof the power supply powering the lamp. The term “lamp” is used to referto any device that may present some form of visible light, such aslighting sources. Flicker may be caused by load fluctuations of a devicenear the flickering device (e.g., the power controller 32 in a dwellingmay cause flicker on lamps in the dwelling). It is desirable to have thepower controller 32 operate such that any voltage fluctuations caused bythe power controller 32 are within an acceptable level such that thereis no visible or limited flicker and/or any flickering is within anacceptable level. Limits on flicker and harmonics are discussed in theInternational Electrotechnical Commission standards IEC 6100-3-3, IEC6100-3-11 and IEC 6100-3-12, the contents of which are hereinincorporated by reference. The power controller 32 may be designed tomeet the limits on flicker and harmonics as discussed in the IEC6100-3-3, IEC 6100-3-11 and IEC 6100-3-12 standards. In general flickermay be measured by perception of light flicker in the long term (P_(lt))which is usually defined as a 2-hour interval or by perception of lightflicker in the short term (P_(st)) which is usually defined as a10-minute interval. Flicker may be measured with commercially availableflicker meters, which return a P_(st) and/or P_(lt) level. Morespecifically, P_(st) may be calculated according to a statisticalprocess over a standardized 10-minute observation interval and P_(lt)may be calculated as the cubic average of several P_(st) values over astandardized two-hour period. For example, a limiting value for P_(st)may be 1.0 and a limiting value for P_(lt) may be 0.65, as specified inthe standards

It should be noted that flicker is not merely a local phenomenon, inother words it does not only affect the loads that connect to the samepower distribution panel in a dwelling. The undesirable phenomena ofsynchronization between multiple power controllers 32, each operating ina respective dwelling, may have an aggregate effect that can produceadditional flicker phenomena at the network level. More generallyflicker may occur such that there is:

-   -   (i) local flicker to other devices on adjacent circuits such as        circuits connected to the same power supply;    -   (ii) flicker at the low voltage transformer of the network which        feeds multiple dwellings; and    -   (iii) flicker at the medium voltage transformer.

Although in the latter two cases there typically would not be anylamp-based devices directly connected to the low voltage transformer orthe medium voltage transformer, voltage fluctuations present at theselocations may propagate through the power network to the devicesindirectly connected thereto.

To avoid flicker at the local level, the power controller 32 could bedesigned such that its electrical energy consumption adjustment strategyand/or modulation strategy would not cause any visible flicker to localdevices or flicker below a specific threshold. For example, if the powercontroller 32 is connected in a residential dwelling to a water heater,the power controller 32 could be designed such that the electricalenergy consumption adjustment strategy does not cause any flicker tolamps within the dwelling or is within an acceptable limit In otherwords, the voltage fluctuations caused by the power controller 32 couldbe designed such that local flicker is below a specific threshold. Thethreshold level of flicker that the power controller 32 is designed toachieve may be a P_(st) value of 1.0 or less and/or a P_(lt) value of0.65 or less. Tests conducted by the present inventors have determinedthat when the power controller 32 is operated according to a schedulesimilar to FIG. 8, in terms of which half cycles are on and which halfcycles are off, visual flicker is maintained within the acceptablelevel, as defined by the aforementioned International ElectrotechnicalStandards IEC 6100-3-3, IEC 6100-3-11 and/or IEC 6100-3-12. It isappreciated that when the power controller 32 is operated according tothe schedule in FIG. 8, in terms of which half cycles are on and whichhalf cycles are off, and additional loads are placed on the powernetwork, that the schedule in FIG. 8 may also assist in maintainingvisual flicker within the acceptable level (e.g., P_(st) value of 1.0 orless and/or a P_(lt) value of 0.65 or less).

To avoid flicker at the low voltage transformer, the power controller 32could be designed such that the electrical energy consumption adjustmentstrategy and/or modulation strategy would not cause any visible flickeror flicker below a specific threshold to devices receiving power via thelow voltage transformer. FIG. 10 illustrates an example of a low voltagetransformer 152 supplying power to three houses 16 ₁ 16 ₂ 16 ₃ eachhaving respective power controllers 32 ₁ 32 ₂ 32 ₃. The electricalenergy consumption adjustment strategy and/or modulation strategy of thepower controllers 32 ₁ 32 ₂ 32 ₃ could be designed such that each of thepower controllers 32 ₁ 32 ₂ 32 ₃ in response to a frequency instabilityevent operate in a manner such that visible flicker or flicker is belowa specific threshold for devices receiving power via the low voltagetransformer 152 (e.g., any lamps in any of the houses 16 ₁ 16 ₂ 16 ₃).More specifically, this may be achieve by designing each of theplurality of power controllers 32 ₁ 32 ₂ 32 ₃ with non-identicalresponses. For example, the first power controller 32 ₁ may have a firstresponse scheme, the second power controller 32 ₂ may have a secondresponse scheme and the third power controller 32 ₃ may have a thirdresponse scheme. Then, in response to a frequency instability event allthree of the power controllers 32 ₁ 32 ₂ 32 ₃ would respectivelydetermine the amount of electrical energy reduction and all three of thepower controllers 32 ₁ 32 ₂ 32 ₃ would then respectively apply theirrespective response schemes. These three response schemes may beaccording to the examples shown in FIGS. 8 and 9. Each of these schemesare designed such the power controllers 32 ₁ 32 ₂ 32 ₃ operate in amanner such that any devices that are supplied power via the low voltagetransformer 152 do not have any visible flicker or have flicker that isbelow a specific threshold. The threshold level of flicker that thepower controllers 32 ₁ 32 ₂ 32 ₃ are designed to achieve may be a P_(st)value of 1.0 or less and/or a P_(lt) value of 0.65 or less.

Turning now to FIG. 12, illustrates an example of a low voltagetransformer 152 supplying power to five houses 16 ₁ 16 ₂ 16 ₃ 16 ₄ 16 ₅where the first three house 16 ₁ 16 ₂ 16 ₃ each have a respective powercontroller 32 ₁ 32 ₂ 32 ₃. In this example, the fourth and fifth house16 ₄ 16 ₅ both do not have a power controller. The electrical energyconsumption adjustment performed by the power controllers 32 ₁ 32 ₂ 32 ₃is designed such that each of the power controllers 32 ₁ 32 ₂ 32 ₃ inresponse to a frequency instability event operate in a manner such thattheir responses are not synchronous, thus keeping flicker below aspecific threshold for all devices receiving power via the low voltagetransformer 152 (e.g., any lamps in any of the houses 16 ₁ 16 ₂ 16 ₃ 16₄ 16 ₅). In other words, by having the response schemes of the powercontroller 32 ₁ 32 ₂ 32 ₃ being according to the examples shown in FIGS.8 and 9 it may be possible to achieve flicker below a specific thresholdfor all dwellings (e.g., the houses 16 ₁ 16 ₂ 16 ₃ 16 ₄ 16 ₅) thatreceive power via the low voltage transformer 152. As discussedelsewhere, the desynchronization of the various power controllers isachieved according to the scheme at FIG. 8, however each powercontroller has a different entry point in the scheme.

To avoid flicker at the medium voltage transformer, the power controller32 could be designed such that the electrical energy consumptionadjustment strategy and/or modulation strategy would not cause anyflicker or flicker below a specific threshold to devices receiving powervia the medium voltage transformer via low voltage transformers. FIG. 11illustrates an example of a voltage down step station 14 that includes amedium voltage transformer 154. The medium voltage transformer 154lowers the electrical voltage from the power plant 10 and is connectedto a plurality of low voltage transformers 152 that supply power todwellings 16 or commercial buildings 18. Similar to the example of thedesign of the power controllers 32 ₁ 32 ₂ 32 ₃ connected to the lowvoltage transformers 152 in order to meet a specific flickerrequirement, the design of the plurality of power controllers that arein the power network that are supplied power by the medium voltagetransformer 154 may be designed such that electrical energy consumptionadjustment strategy and/or modulation strategy of these plurality ofpower controllers would not have any visible flicker or have flickerthat is below a specific threshold for lamp-based devices supplied powervia the medium voltage transformer 154. This may be achieved bydesigning the schemes of the plurality of power controllers in thenetwork such that some of the power controllers have non-identicalresponse such that the plurality of power controllers collectivelyachieve the flicker threshold for devices receiving power via the mediumvoltage transformer 154. As shown in the example of FIG. 12, in theplurality of houses 16 ₁ 16 ₂ 16 ₃ 16 ₄ 16 ₅ only some of the house (16₁ 16 ₂ 16 ₃) have power controllers (32 ₁ 32 ₂ 32 ₃) and similar to theexample of the low voltage transformer 152 (discussed above) theresponse schemes of the power controller 32 ₁ 32 ₂ 32 ₃ may beingaccording to the examples shown in FIGS. 8 and 9 such that it may bepossible to achieve flicker below a specific threshold for all dwellings(e.g., the houses 16 ₁ 16 ₂ 16 ₃ 16 ₄ 16 ₅) that receive power via themedium voltage transformer 154.

It should be appreciated that if the power controllers 32 ₁ 32 ₂ 32 ₃each have the table of FIG. 8 and enter the table of FIG. 8 (or similar)at different entry points, in response to a frequency imbalance event,that this may assist in minimizing flicker and meeting the flickerdesired limits as discussed above (e.g., P_(st) value of 1.0 or lessand/or a P_(lt) value of 0.65) and/or the limits in the InternationalElectrotechnical Standards IEC 6100-3-3, IEC 6100-3-11 and/or IEC6100-3-12.

Although in the examples above the modulation strategy reduceshalf-cycles, in other embodiments the modulation strategy could includereducing full-cycles.

In theory, the voltage waveform applied to the electric load 34 may bereduced in various ways, other than reducing half-cycle of the voltagewaveform. However, the practical reality is that reducing only partialhalf-cycles may introduce undesirable harmonics. Although it ispreferred to reduce the voltage waveform applied to the electric load 34by reducing half-cycles, some other possible techniques are as follow:

(i) Random Cycle Delay

FIG. 13 illustrates a specific and non-limiting example where the ACpower grid has three power controllers 32 ₁ 32 ₂ 32 ₃ and threeelectrical loads 34 ₁ 34 ₂ 34 ₃, respectively. It is appreciated thatthe number of electrical loads illustrated in this example is forillustration purposes only and the number and type of loads on the powernetwork may vary. In the subsequent examples, the reference (a) is usedto refer to the voltage wave form supplied from the first powercontroller 32 ₁ to the first electric load 34 ₁, the reference (b) isused to refer to the voltage wave form supplied from the second powercontroller 32 ₂ to the second electric load 342 and reference (c) isused to refer to the voltage wave form supplied from the third powercontroller 32 ₃ to the third electric load 34 ₃.

An example of adjusting the voltage waveform is the introduction of arandom cycle delay in the response of each power controller 32 ₁. Inthis case, after the degree of electrical consumption reduction isdetermined (step 102), then the power reduction strategy (step 104)includes determining a delay and waiting for the duration of the delayperiod prior to initiating the response. The delay may be an integerbased delay corresponding with a number of cycles (e.g., X full cycledelay; where X is a non-negative integer, such as 0, 1, 2,. . . ). Anupper limit may be set for the maximum number of cycles to delay. Forexample, it may be desirable for all of the power controllers 32 ₁ 32 ₂32 ₃ in the set to initiate load reduction within a window or a certainnumber of cycles or seconds (e.g., 60 cycles or 1 seconds). In othercases, the delay may be a time period which does not necessarilycorrespond to a full number of cycles.

FIG. 14 illustrates an example of the three voltage wave forms (a) (b)(c) applied to the respective electrical loads 34 ₁ 34 ₂ 34 ₃. In thisexample the delay of the first power controller 32 ₁ is 1 cycle, thedelay of the second power controller 32 ₂ is 0 cycles and the delay ofthe third power controller 32 ₃ is 2 cycles. Since the various powercontrollers 32 ₁ 32 ₂ 32 ₃ initiate their response at different times,this approach may reduce unwanted distortions on the power grid.

The delay may be determined by a random number generator. The CPU 42 mayexecute instructions stored on the memory 44 of the power controller 32to randomly generate a non-negative integer. The random number generatormay be based on a seed value or maybe a quasi-random number generator.In other cases, the delay may be preset in each power controller 32 ₁ 32₂ 32 ₃. In other words, each of the power controller 32 ₁ 32 ₂ 32 ₃ ispreset at the time of manufacturing or prior to the installation with astatic delay that varies among the set of power controllers 32 ₁ 32 ₂ 32₃. In yet other cases, each of the power controllers 32 ₁ 32 ₂ 32 ₃ ispreset at the time of manufacturing or prior to the installation with astatic seed value that varies among the set of power controllers 32 ₁ 32₂ 32 ₃ and the seed value of each of the power controller 32 ₁ 32 ₂ 32 ₃is used by the random number generator to then generate a quasi-randomdelay number.

(ii) Random Offset

Another example is the introduction of a random offset. To introduce therandom offset, a delay time (dt) and a load reduction time (lrt) aredetermined. The determination of the delay time (dt) and the loadreduction time (lrt) are associated with the determined amount ofreduction of the electrical consumption (from step 102), as thedetermined amount of reduction of the electrical consumption may place alimit of a suitable value for the delay time (dt) and determined loadreduction time is dependent on the determined value of (dt) along withthe determined amount of load reduction.

FIGS. 15A and 15B illustrate examples of voltage waveforms applied tothe electrical load 34 based on determining the amount of reduction ofthe electrical consumption and then determining the delay time (dt) andthe load reduction time (lrt) to achieve the determined amount ofreduction of the electrical consumption. The delay time (dt) may be arandomly generated number in the interval of 0 to 0.5*C/f, where f isthe fixed frequency (usually 50 or 60 Hz) and C is the percentage ratedvalue (in the decimal number equivalent in the range of 0 to 1) of thecontrolled load as determined in step 102. Once the delay time (dt) isdetermined, a load reduction time (lrt) may be determined based on thedelay time (dt), where the load reduction time (lrt) starts after thedelay time (dt) and is the duration of time where the load is reduced tozero. The determination of the load reduction time (lrt) may be based oncomputing a mathematical formula (e.g., based on the area under thevoltage curve) or using the determined delay time (dt) to look-up theload reduction time (lrt) in a lookup, such as the table shown in FIG.16.

By way of a specific and non-limiting example, if it is determined thatthe frequency rate of variation is −0.25 Hz/s then the power controller32 may then use a look-up table and determine that the reduction of theelectrical consumption should be 50%. Based on this desired amount ofreduction of the electrical consumption a delay time (dt) can bedetermined, a random number generator may be used to generate the delaytime (dt) where the output parameters of the random number generator areconfigured to be between 0 and 0.25 of the cycle. Then the random numberis generated and in this case equals to 0.25 of the cycle (e.g., 4.166ms, assuming a 60 Hz waveform). The load reduction time (lrt) isdetermined and in this case is done by looking-up the load reductiontime (lrt) based on the delay time (dt) in a lookup table (e.g., seeFIG. 16). The power controller 32 then steps in via the powerelectronics 46 to reduce the electrical load 34 according to thedetermined amount of reduction of the electrical consumption as shown inFIG. 15A. Although not illustrated, the power controller 32 may continueto implement the reduction of the electrical consumption as shown inFIG. 15A for multiple cycles until a new frequency rate of variation isdetermined and the process 100 is repeated for the new frequency rate ofvariation.

The lookup table may include a series of lookup tables for a series ofdifferent controlled load values. As shown in FIG. 16, a lookup tablefor a series of different delay times and load reduction times isprovided for a controlled load of 50%. Once the amount of reduction ofelectrical energy is determined, the appropriate controlled load tablemay then be used to randomly select an entry in the table to obtain thedelay time (dt) and load reduction time (lrt).

Alternatively a load conduction time may be determined instead of a loadreduction time (lrt), where the load conduction time is the time theload is conducting and is associated with the delay time (dt) and thedetermined amount of reduction of electrical energy consumption (fromstep 102) and is determined in a similar manner to that of the loadreduction time (lrt).

(iii) Random Reduction of Full Cycles

Another example is to determine a number of full (or half-cycles, asdiscussed elsewhere in this document) to reduce in a window and reduce arandom set of cycles in the window that correspond to the determinednumber of cycles to be reduced. The window size may vary depending onimplementation. By way of example, a suitable window time may beanywhere in the range of 0.1 to 10 seconds.

FIG. 19 illustrates the first 6 cycles of a set of voltage waveforms forthe set of power controllers 32 ₁ 32 ₂ 32 ₃. As shown, each powercontroller 32 in the set of power controllers 32 ₁ 32 ₂ 32 ₃ generates aset of cycles in a window time. In this example, there are 6 cyclesillustrated, and as such the window time is 6 cycles. Also, in thisexample it is assumed that the determined amount of electricalconsumption reduction results in a controlled load of 66.67% of therated value. Each of the power controllers 32 ₁ 3 ₂₂ 32 ₃ determinesthat 2 of the 6 cycles in the window should be reduced based on thisload reduction and then determines a random set of cycles that should bereduced to achieve this desired controlled load. As shown, powercontroller 32 ₁ determines that cycles 2 and 6 should be reduced, powercontroller 32 ₂ determines that cycles 1 and 2 should be reduced andpower controller 32 ₁ determines that cycles 2 and 5 should be reduced.

The power reduction process can also be implemented as follows. Afterthe degree of electrical consumption reduction is determined (step 102),the electrical consumption process (step 104) includes first determiningthe number of cycles to be reduced. As shown in FIG. 17, a lookup tablemay be used to determine the number of cycles to be reduced. In theexample given in FIG. 17, the process operates on a 1 second window. Arandom number generator may be used to determine which cycles in thewindow should be reduced. For example, in a 1 second window there are 60cycles (assuming a 60 Hz signal), and if it is determined that theelectrical consumption should be reduced by 50% then 30 of the 60 cyclesshould be reduced and the random number generator would generate a setof 30 unique numbers in the range of 1 to 60 and then the cyclescorresponding to the generated set of 30 unique numbers would then bereduced. Alternatively, a binary number generator may be used togenerate an ordered sequence of binary numbers (i.e., 0 or 1) thatcorrespond in length with the number of cycles in the window and wherethe distribution of the sequence has a set number of 0's that correspondwith the number of cycles to be reduced. The ordered sequence of binarynumbers can then be correlated against the cycles in the window.

For each subsequent 1 second window, the process is repeated. One optionis to keep reducing the same cycles determined previously.Alternatively, the cycles to be reduced are recomputed at each window.

(iv) Random Reduction of Full Cycles Based on Predefined Sequences

A further example, which is similar to the example above, is providedfor an electrical consumption reduction process. In this case, insteadof using the random number generator for determining which cycles toreduce, a lookup table is provided which includes a set of predeterminedcycles to reduce. Then a random number generator is used to determinewhich entry of the table to use. FIG. 18 illustrates an example of alookup table that may be used based on the determined amount ofelectrical consumption reduction and window time. A random numbergenerator may be used to select which entry of the table shown in FIG.18 to use. It is appreciated that the lookup table will depend on thewindow time and the table illustrated is for a 1 second window time.

v) DC Conversion

Another example, for pseudo randomizing the process of reducing the loadis in the case where the AC voltage is rectified to a DC voltage. The DCvoltage can then be used to power the load and can be modulatedaccordingly.

FIG. 20A shows an example of a DC voltage that has been rectified froman AC voltage by the power controller 32. FIG. 20B shows an example of aDC voltage that has been rectified from an AC voltage by the powercontroller 32 and where the DC voltage is controlled by pulse widthmodulation to achieve the target RMS voltage.

The window time may correspond in time with one or more full cycles ofthe AC signal that is rectified to DC. That is, the window time may be apositive integer multiple of the cycle time of the AC signal.

It is appreciated that in this example, the power electronics 46 of thepower control device 32 further include electronics for rectifying an ACsignal to a DC signal.

Certain additional elements that may be needed for operation of someembodiments have not been described or illustrated as they are assumedto be within the purview of those of ordinary skill in the art.Moreover, certain embodiments may be free of, may lack and/or mayfunction without any element that is not specifically disclosed herein.

Any feature of any embodiment discussed herein may be combined with anyfeature of any other embodiment discussed herein in some examples ofimplementation.

Although various embodiments and examples have been presented, this wasfor the purpose of describing, but not limiting, the invention. Variousmodifications and enhancements will become apparent to those of ordinaryskill in the art and are within the scope of the invention, which isdefined by the appended claim(s).

1. A method for reducing aggregate load creep-up in a power distributionnetwork which supplies an aggregate load including a plurality ofindividual loads controlled by respective power control devices, thepower control devices being responsive to a power generation deficit inthe power distribution network to reduce an electrical consumption ofthe respective loads to a level selected according a magnitude of theimbalance, the method including an act performed by each of theplurality of the power control devices subsequent the reduction ofelectrical consumption to the selected level, the act includingprogressively reducing the electrical consumption below the selectedlevel to reduce a likelihood of aggregate load creep-up.
 2. A method asdefined in claim 1, wherein a reduction of the electrical consumption tothe selected level is performed according to a first average reductionrate, the act of progressively reducing the electrical consumption beingperformed according to a second average reduction rate, the first ratebeing higher than the second rate.
 3. A method as defined in claim 1,wherein a reduction of the electrical consumption to the selected levelis performed during a first time period, the act of progressivelyreducing the electrical consumption being performed during a second timeperiod, the first time period being shorter than the second time period.4. A method as defined in claim 1, wherein the act of progressivelyreducing the electrical consumption below the selected level isautonomously performed by each of the plurality of the power controldevices.
 5. A method as defined in claim 1, wherein the act ofprogressively reducing the electrical consumption below the selectedlevel is initiated when the power generation deficit reaches a nadir. 6.A method as defined in claim 1, wherein each of the power controldevices is responsive to a frequency of the electrical energy in thepower distribution network to determine the magnitude of the powergeneration deficit.
 7. A method as defined in claim 1, wherein the actof progressively reducing the electrical consumption being performedaccording to an average reduction rate, the average reduction rate beingrelated to an inertia of the load controlled by the power controldevice.
 8. A method as defined in claim 1, wherein the act ofprogressively reducing the electrical consumption being performedaccording to an average reduction rate, the method including selectingthe average reduction rate among a plurality of reduction rates andprogressively reducing the electrical consumption according to theselected reduction rate.
 9. A method as defined in claim 1, wherein theact of progressively reducing the electrical consumption being performedaccording to an average reduction rate, the method including determiningthe average reduction rate according to characteristics of therespective load controlled by the power controlled device andprogressively reducing the electrical consumption of the load accordingto the determined reduction rate.
 10. A power control device forreducing aggregate load creep-up in a power distribution network whichsupplies an aggregate load including a plurality of individual loads,the power control device configured for controlling an electricalconsumption of a respective load among the plurality of individualloads, the power control device comprising: a. one or more processors;b. a machine readable storage encoded with software for execution by theone or more processors, the software defining an electrical consumptioncontrol logic operative for: i. in response to a power generationdeficit in the power distribution network to reduce an electricalconsumption of the respective load to a level selected according amagnitude of the imbalance; ii. subsequent the reduction of electricalconsumption to the selected level, progressively reducing the electricalconsumption below the selected level to reduce a likelihood of aggregateload creep-up when a plurality of the power control devices autonomouslycontrol the electrical consumption of the respective ones of theindividual loads.
 11. A power control device as defined in claim 10,wherein a reduction of the electrical consumption to the selected levelis performed according to a first average reduction rate, the controllogic progressively reducing the electrical consumption below theselected level according to a second average reduction rate, the firstrate being higher than the second rate.
 12. A power control device asdefined in claim 10, wherein a reduction of the electrical consumptionto the selected level is performed during a first time period, thecontrol logic progressively reducing the electrical consumption belowthe selected level during to a second time period, the first time periodbeing shorter than the second time period.
 13. A power control device asdefined in claim 10, wherein the control logic is configured forprogressively reducing the electrical consumption below the selectedlevel autonomously of another power control device operating in thepower distribution network.
 14. A power control device as defined claim10, wherein the control logic is configured for progressively reducingthe electrical consumption below the selected level when the powergeneration deficit reaches a nadir.
 15. A power generation device asdefined in claim 10, wherein the control logic is responsive to afrequency of the electrical energy in the power distribution network todetermine the magnitude of the power generation deficit.
 16. A powercontrol device as defined in claim 10, wherein the progressive reductionof the electrical consumption being performed according to an averagereduction rate, the average reduction rate being related to an inertiaof the load controlled by the power control device.
 17. A power controldevice as defined in claim 10, wherein the control logic is configuredof progressively reducing the electrical consumption according to anaverage reduction rate, the control logic configured for selecting theaverage reduction rate among a plurality of reduction rates andprogressively reducing the electrical consumption according to theselected reduction rate.
 18. A power control device as defined in claim10, wherein the control logic is configured for progressively reducingthe electrical consumption according to an average reduction rate, thecontrol logic being configured for determining the average reductionrate according to characteristics of the respective load controlled bythe power controlled device and progressively reducing the electricalconsumption of the load according to the determined reduction rate. 19.A power control device as defined in claim 10, including powerelectronics responsive to the control logic to vary an RMS voltage tothe respective load to adjust the electrical consumption of the load.20. A power control device for use in a power distribution networksupplying electrical energy to a plurality of individual loads, thepower control device for use in controlling an electrical consumption ofan individual load among the plurality of individual loads, the powercontrol device comprising: a. an input for receiving informationidentifying a presence of a power generation deficit in the powerdistribution network; b. a control entity; c. power electronics forregulating a supply of electrical energy from the power distributionnetwork to the individual load, the supply of electrical energy havingsinusoidal voltage cycles, each sinusoidal voltage cycle including apositive half-cycle and a negative half-cycle, the control entity,configured for: i. in response to a power generation deficit in thepower distribution network, selecting a reduced non-nil electricalconsumption level for the individual load among a plurality of possiblereduced non-nil electrical consumption levels; ii. determining acombination of half-cycles to block from the electrical energy suppliedto the individual load corresponding to the selected reduced non-nilelectrical consumption level; iii. control the power electronicsaccording to the determining to achieve the selected reduced non-nilelectrical consumption level. 21.-70. (canceled)